1. Field of the Invention
Apparatuses and methods consistent with the present invention relate to an image signal processing, and more particularly, to image signal processing in which an input analog image signal is converted into a digital image signal compliant with a system standard.
2. Description of the Related Art
An analog image signal is generally classified into a composite video blanking and Sync (CVBS) signal, a super (S) signal, and a component signal. The CVBS signal combines a luminance (Y) signal and a chrominance (C) signal and is transmitted according to one of a Phase Alternation by Line System (PAL), a National Television System Committee (NTSC), and Sequential Color with Memory (SECAM).
FIG. 1 is a view illustrating a waveform of a related art CVBS signal.
The waveform illustrated in FIG. 1 represents a horizontal line of one frame of an NTSC image signal. The horizontal line comprises a horizontal synchronization signal (H-SYNC), a color burst signal (COLOR BURST), and a color signal (COLOR) corresponding to values of pixels of one horizontal line.
The COLOR BURST is a reference signal added for a demodulation of a color when a color image signal is transmitted, and is often referred to as a “color subcarrier reference signal.” The COLOR BURST is transmitted in a horizontal blanking period where there is no COLOR, and is used at a replay side to demodulate the COLOR based on the waveform.
In NTSC, amplitude of a normal COLOR BURST without a distortion is 40 IRE (140 IRE=1 V as that of the H-SYNT), and a period of a normal COLOR BURST is approximately 3.58 MHz.
Most of the image signal processors execute an automatic chroma gain control (ACC) using a COLOR BURST to compensate distortion of an image signal when the size of the image signal is distorted in transmission and reception. That is, the size of a COLOR is adjusted to that of a reference COLOR corresponding a COLOR BURST.
Amplitude of a COLOR BURST in a normal image signal is 40 IRE, but amplitude of COLOR BURST in a distorted image signal is lower or higher than 40 IRE. If the amplitude of the COLOR BURST is lower than a reference level, the amplitude of the COLOR is amplified to meet the reference level of the COLOR corresponding to the amplitude of the normal COLOR BURST. On the other hand, if the amplitude of the COLOR BURST is higher than a reference level, the amplitude of the COLOR is attenuated to attain to the reference level of the COLOR corresponding to the amplitude of the normal COLOR BURST.
When the size of a COLOR is lower or higher than that of a reference COLOR, the process of ACC includes multiplying the size of the COLOR by a difference between the size of the COLOR and the size of the reference COLOR to compensate the distorted COLOR. The ACC gain is the multiplier of the multiplication. The multiplication is reiterated by feedback until the ACC gain converges into 1. Accordingly, if a difference is large between the size of a COLOR of an input CVBS signal and the size of a reference COLOR, the ACC takes long amount of time. As a result, transient phenomenon appears on a screen, or a brightness varies slow, and an image quality is deteriorated.